This invention relates generally to the protection of waterways such as harbors, rivers, lakes, etc. and, more particularly, to an acoustic surveillance system designed to track a swimmer or underwater vehicle or target or object or the like that could pose a danger to ships and other installations in a harbor.
Ships and other installations in a waterway or other water source are vulnerable to interlopers who can penetrate into the area by swimming undetected into the area. A defense against such a threat is a surveillance system that continuously monitors the region for swimmers. Electromagnetic waves do not travel far in water. Similarly the effectiveness of using light to probe the water is limited because of the poor visibility. On the other hand sound can travel far in water. Hence the swimmer detection systems currently available are generally sonar systems.
Current swimmer detection systems are generally active systems, i.e. they transmit a high frequency acoustic pulse into the water. The acoustic pulse, when incident on an object, reflects some of the incident acoustic energy. This reflected energy is sensed by a sonar system and the sonar system then makes the decision whether the object is a threat or not. Once a threat is detected this information is passed on for action to be taken against the threat. Some of the basic requirements for such a system are:                1. The range for detection of an object such as a swimmer should be as large as possible, but must be sufficient to react to the threat. The detection range is a function of the characteristics of the acoustic pulse (frequency, duration, signal type, etc.), the acoustic source level, the directivity of the source, the ambient noise, the strength of interfering returns from false targets, and the strength of the target return.        2. Robust signal processing schemes are needed that analyze the reflected acoustic signal and decide whether a threat exists or not.        3. The area to be covered by the system is generally large, and, due to limitations in detection range, will therefore require a number of units to be deployed. To limit the overall cost of the installation, the cost of each individual unit has to be low.        4. The system has to be capable of operating unattended for an extended period of time in the hostile ocean environment.        5. The system would require periodic maintenance. Therefore ease of maintenance is an important consideration.        6. Generally, the system should be capable of being mounted on different platforms.        
Most swimmer detection systems that are currently available in the market use phased arrays. A phased array is an array of transducers which may be line arrays, a planar array, or a volume array. The transducer is commonly made of piezo-electric material, which has the property of converting an electrical signal to a pressure (acoustic) signal or acoustic wave and vice versa. By suitably varying the phase of the electrical input into each of the transducer, the acoustic beam emanating from this array can be made to transmit the energy in a particular direction. Similarly the reflected energy received by these transducers can be processed to discern the direction from which the energy is coming. These phased array systems are quite complex in terms of the hardware that they require and in terms of the software needed to do the phasing and analysis. The result is a very expensive system.
Considering that there are a large number of waterways that need to be overseen, there is a clear need to develop a low cost system that will meet the requirements listed above.